Intrinsic cardiac pulses may be measured using an electrode placed in or in the vicinity of a heart and an example of such pulses for a healthy patient is shown in FIG. 1A. For each heartbeat, the pulses comprise a wave labelled P which causes the atria to contract, and a complex wave form labelled QRST associated with the contraction of the ventricles. A pacemaker circuit comprises a pulse generator connected to a stimulating electrode embedded in the patient's atrium or ventricle and a reference electrode positioned outside the heart in a unipolar configuration, or on the same lead within the heart in a bipolar system. The generator applies stimulus pulses across these two electrodes which replaces the evoked pulses. During each stimulus pulse, electrical charge is stored in the interface between the stimulus electrode and the myocardium, and the charge then decays upon termination of the stimulus pulse. An example of the waveform appearing at the stimulus electrode is shown in FIG. 1B in which each stimulus pulse is denoted by St. As may be seen, the charge which is stored and its subsequent decay masks the evoked heart response. However, it is useful to detect these evoked pulses as they may be used both for diagnosis and for modifying the operation of the pacemaker circuit.